1. Field of the Invention
The present invention is related to line powering for single twisted pair for use in communication systems. In particular, the present invention relates to method and apparatus for alternately delivering negative and positive polarity dc voltages across a single twisted pair.
2. Description of the Related Art
Techniques for delivering dc voltage over a single twisted pair commonly known as line powering is known. Typically, line powering is used for providing power to remote terminals in a Digital Subscriber Loop (DSL) system or a T1/E1 system where a central office provides power to operate a remote terminal and where the central office is connected to the remote terminal via a single twisted copper pair in the case of a DSL system, and by two twisted copper pairs in the case of a T1/E1 system.
For safety reasons, there are restrictions on the level of permissible dc voltages over the twisted copper pair, and several class ratings are assigned for the various operating voltages. For example, a Class A1 voltage rating limits the permissible current to less than 0.15 mA for magnitude of dc voltages between 30 and 200 volts. However, there is no current limit imposed for magnitude of dc voltages below 30 volts. A Class A2 voltage rating limit is defined for current less than 5 mA for the magnitude of dc voltage range defined between 80 to 200 volts. For magnitude of dc voltages below 80 volts, there is not current limit. Finally, Class A3 rating is defined for the magnitude of dc voltages between 140 to 200 volts whose permissible current can go as high as 10 mA. For the magnitude of dc voltages less than 140 volts under the Class A3 rating, no current limit is imposed. As mentioned above, the current limitation under the various ratings is imposed as a safety precaution for telephone company personnel if they come into direct contact with either the Ring or the Tip terminal which comprise the single twisted copper pair coupling the central office to the remote terminal.
Asymmetric Digital Subscriber Line (ADSL), High-speed Digital Subscriber Line (HDSL), and other types of DSL technologies and T1/E1 system available today are presently pushing for distances to as much as 100 Kft using line powering. To reach such distances, the magnitude of line powering voltages between the Tip terminal and the Ring terminal (Tip-to-Ring) as high as 400 volts is necessary. In order to provide such line powering voltages and comply with the safety class rating discussed above, a +/xe2x88x92200 volts is used between the Tip terminal and ground terminal (Tip-to-Ground), and between the Ring terminal and the ground terminal (Ring-to-Ground) which would fall under the Class A2 rating.
One significant problem with using the above-discussed voltages is that whenever a positive voltage is applied to the terminals, those terminals are prone to corrosion. An electrically conducting material such as metal wires are susceptible to oxidation which results from a difference in the electrical potential of the surrounding ground and the conducting material. A known approach to compensate for the deterioration from oxidation is cathodic protection whereby a small electric charge is applied between the conducting material to be protected and the ground to oppose the flow of electrons such that the applied voltage is greater than the oxidation voltage. The negative charge thus applied to the transmission lines for line powering therefore provides cathodic protection from deterioration of the transmission lines from oxidation. Moreover, due to this corrosion effect on the transmission lines, customers are not likely to install systems that use positive voltages, and therefore, the suppliers of the equipment to these customers must limit the voltages to negative dc voltage only. Naturally, such limitation is a severe trade-off for the distance that can be reached with the line powered single twisted cable pair.
In view of the foregoing, there are provided method and apparatus for alternately delivering positive and negative dc voltages to a single twisted pair to minimize the corrosive effect of the positive dc voltage.
A system for providing line powering to a twisted pair configured to transmit and/or receive signals in accordance with one embodiment of the present invention includes a power supply for generating power signals; a voltage alternating switch circuit operatively coupled between the power supply and a twisted pair; and a controller coupled to the voltage alternating switch circuit for providing control signals thereto to control line powering to the twisted pair.
A method for providing line powering to a twisted pair configured to transmit and/or receive signals in accordance with another embodiment of the present invention comprises the steps of: discharging line voltages from a plurality of transmission lines; alternating the voltage signals to alternately provide to the respective transmission lines; and applying the alternated voltage signals to each transmission line such that each polarity of the voltage signal applied to each transmission line is reversed.
A method for providing line powering to a twisted pair configured to transmit and/or receive signals in a communication system in accordance with yet another embodiment of the present invention includes the steps of: monitoring the telephone lines coupled to the communication system to determine whether they are on hook or off hook; discharging line voltages from a plurality of transmission lines; alternating the voltage signals to alternately provide to the respective transmission lines; and applying the alternated voltage signals to each transmission line such that each polarity of the voltage signal applied to each transmission line is reversed.
An apparatus for alternately providing signals in accordance with yet one embodiment of the present invention includes a power supply configured to provide a plurality of signals each having a different polarity; a plurality of terminals; a switching network including: a resistance; a first pair of switches operatively coupled between said power supply and said plurality of terminals, and a second pair of switches operatively coupled between said resistance, said power supply and said plurality of terminals; and a controller coupled to said switching network to control said switching network in accordance with a predetermined timing such that each of said terminals alternately receive said different polarity signals in accordance with said predetermined timing.
An apparatus for alternately providing signals in accordance with yet another embodiment of the present invention includes a power supply configured to provide a plurality of signals each having a different polarity; a plurality of terminals; a switching network including: a resistance; a first pair of switches operatively coupled between said power supply and said plurality of terminals, and a second pair of switches operatively coupled between said resistance, said power supply and said plurality of terminals; and a controller coupled to said switching network to control said switching network in accordance with a predetermined timing such that each of said terminals alternately receive said different polarity signals in accordance with said predetermined timing; wherein said controller controls said switching network such that during a first portion of said predetermined timing, said power supply is coupled to said plurality of terminals and during a second portion of said predetermined timing, said resistance is coupled to said plurality of terminals.
A method of alternately providing signals in accordance with still another embodiment of the present invention includes the steps of: providing a plurality of signals each having a different polarity from a signal source; selectively coupling a first pair of switches between said signal source and a plurality of terminals; selectively coupling a second pair of switches between a resistance, said signal source and said plurality of terminals; and providing a plurality of control signals such that said first and second pairs of switches are selectively coupled in accordance with a predetermined timing and each of said terminals alternately receive said different polarity signals in accordance with said predetermined timing.
These and other features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings.